Re: More Guile 1.8 code failing in 2.x

[mark@markwitmer.com]

Richard Shann <richard.shann@virgin.net> writes:

> We have one more construct being refused as we upgrade to guile 2.0 in
> GNU/Denemo
>
> (if (not (defined? 'ToggleTripleting::InsideTriplet))
>     (define ToggleTripleting::InsideTriplet #t))
>
> It is intended to set up a flag which toggles between true and false on
> each call. If already set up, the flag is not altered.
>
> Apparently (I haven't been able to check) Guile 2.0  gives an error.
>
> #f definition in expression context, where definitions are not
> allowed, ((line . 2) (column . 4) (filename . #f)) (define
> ToggleTripleting::InsideTriplet #t) #f)
>
> Can anyone suggest what could replace that? I would prefer to to replace
> it with something that has the same semantics, rather than setting up
> some alternative method of handling the situation - registering all such
> variables on program startup, or some such - as at the moment this code
> only evaluated if used and doesn't require any mechanism to get
> initialized, other than this construct.
>
> Richard Shann
>
>
>
>

Hi Richard,

You can probably just replace your code with

(define-once ToggleTripleting::InsideTriplet #t)

(define-once ...)  works like defvar in Common Lisp and will not
redefine a variable once it has been defined.  You can read about it at
the bottom of the node "Top Level Variable Definitions" in the Guile
manual.

I'm still learning the ins and outs of procedural macros, but I think
that if you need to include (define ...) in a conditional like that for
more complicated purposes, you can use syntax-case like this:

(define-syntax conditional-define
  (lambda (x)
    (syntax-case x ()
      ((conditional-define arg val)
       (let ((arg-datum (syntax->datum #'arg)))
	 (if (not (defined? arg-datum))
	     #'(define arg val) 
             #'*unspecified*))))))

-- 
Mark Witmer